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How do you find center of lift/gravity for custom scratch builds?

#1
Inspired by the article about scratch building without wasting foam board, I am trying to build a larger version of Nutball with swappable holes for the power pod. I am also using larger sized elevons. Not having to print out the design, join it together with tape, and transfer it to the foam board feels liberating.

Now that I have already cut up the custom design in the foam board, I am faced with the problem of finding the center of lift for the modified Nutball. I am assuming that the center of lift should be the same as center of gravity.

How does anyone determine center of lift for any plane? Is there an easy trick to finding the where to hold the plane to know if it is tail heavy or nose heavy?

Since the planes are symmetrical on both sides, I understand that we need to concentrate on length wise position.
How will this change for planes that are not symmetrical? Say, if you are flying someone's side profile on a nutball style design..how do you find its center?
 

adamd

skunkworx hobbies
#2
there are alot of good cg calculators on the net. i find with flat wing designs(profile fomies) the cg is alot more forward then a conventional wing. the good glide test never hurts too, play around with your battery location to change the cg then just toss it on a bed or into some pillows. i find once you build a few models you get a feel of where a cg and cl should be. then its just playing around to find a sweet spot
 

rcspaceflight

creator of virtual planes
#3
The only practical way is to to glide tests. Make the CG where you think it is, then throw the plane and see how well it glides. If the nose seems to tip up, then it's tail heavy. If the nose dives down, then it's nose heavy. Even if you think you have it close to correct, still move around the weight (battery) and see what it does with different CG locations.

CG being center of gravity which is the same as center of lift.
 

adamd

skunkworx hobbies
#4
cg and cl are different but you tend to fly wing them both at the same point, some of my 3d guys have the cg aft of the cl.
the cl of a wing is determined by the wing design and stays for the most part in one place while the cg is adjustable and determined by airframe and component weights. the cg can be moved where you like for best balance around your cl.
 

earthsciteach

Moderator
Moderator
#5
Rule of thumb is that the center of lift is at mid-chord of the wing. That's easy for a conventional wing, but can be more complicated with weird geometries. The cg should be slightly forward of the cl (in general) but there can be more to it than that. The neutral point of an aircraft takes into account the moment of the horizontal tail surface. If you have a "flying" tail, the cg might be behind the cl. Canard aircraft sort of work the same way.
 
#6
Thanks to all for the feedback. Based on this information, I now understand the following:

Since the center of lift (CL) stays the same, but center of gravity (CG) can be shifted, the best way to find the CL is to move the CG, chuck the aircraft into a soft surface/bed, and see if it is nose heavy or tail heavy. When finally it flies/glides straight, then the points on which the plane balances marks the CL. Does this make sense?

Based on other form posts, the CG should be close by but just a little in front of the CL for the aircraft to fly good and avoid stalls. So if my understanding is right, I should be able to figure out the CL by trial and error.